First recognized in mid-March 2003, Severe Acute Respiratory Syndrome (SARS) was
successfully contained in less than four months. On 5 July 2003, WHO reported that the last human
chain of transmission of SARS had been broken.

Much has been learned about SARS, including its causation by a new coronavirus (SARS-CoV);
however, our knowledge about the ecology of SARS coronavirus infection remains limited. In the
post-outbreak period, all countries must remain vigilant for the recurrence of SARS and maintain
their capacity to detect and respond to the re-emergence of SARS should it occur. Resurgence of SARS
remains a distinct possibility and we need to be prepared.

For the third edition, most chapters have remained unchanged, with two exceptions: the Virology
section has been updated and the chapter entitled SARS Treatment
has been completely rewritten by
Loletta So, Arthur Lau, and Loretta Yam from the Division of Respiratory and Critical Care Medicine,
Department of Medicine, Pamela Youde Nethersole Eastern Hospital, Hong Kong SAR, PR China. In the
event of a new SARS outbreak, we shall have to rely on existing treatment modalities. These have now
been brilliantly overviewed by our new colleagues.

Just over five months ago, SARS started to spread around the world.
It is the first major new infectious disease of this century, unusual in its high morbidity and
mortality rates, and it is taking full advantage of the opportunities provided by a world of
international travel. At the time of this writing, more than 8,000 persons with probable SARS have
been diagnosed; 812 patients have died. Fortunately, one by one, the outbreaks in the initial waves
of infection have been brought under control.

SARS demonstrates dramatically the global havoc that can be wreaked by a newly
emerging infectious disease. SARS was capable of bringing the healthcare system of entire areas to a
standstill, striking nurses, doctors and other medical personnel: human resources vital for disease
control. Surgery and vital treatments for patients with serious conditions had to be postponed; care
in emergency rooms was disrupted. A significant proportion of patients required intensive care, thus
adding to the considerable strain on hospital and healthcare systems. Hospitals, schools, and
borders were closed. The economic impact on individuals was profound, affecting tourism, education
and employment.

The disease has several features that make it a special threat to international
public health. There is no vaccine or treatment, and health authorities have to resort to control
tools dating back to the earliest days of empirical microbiology: isolation, infection control and
contact tracing.

The response of the scientific community to the new health threat was immediate
and breath-taking. The etiologic relationship between a previously unknown coronavirus and SARS was
established one month after the WHO issued a global alert and called upon 11 leading laboratories in
9 countries to join a network for multicenter research into the etiology of SARS and to
simultaneously develop a diagnostic test. The early recognition of the etiologic agent has made the
virus available for investigation of antiviral compounds and vaccines.

Experience with SARS has shown that, with strong global leadership by the WHO,
scientific expertise from around the world can work in a very effective, collaborative manner to
identify novel pathogens.

SARS has demonstrated how the world
can come together in scientific collaboration, and what the power of the Internet is. This outstanding effort limited the potentially explosive spread of the outbreak.

Some hope exists that the disease might be contained, but much about SARS remains
unknown. How important are animals in its transmission? Will SARS return with a stronger force next
year? What are the host or virus factors responsible for the "superspreader" phenomenon, in which a
single patient may infect many people through brief casual contact or possibly environmental
contamination?

At this moment, a global epidemic of the magnitude of the 1918-19 influenza
pandemic appears unlikely. However, development of effective drugs and vaccines for SARS is likely
to take a long time. If SARS is not contained, the world will face a situation in which every case
of atypical pneumonia, and every hospital-based cluster of febrile patients with respiratory systems
will have the potential to rouse suspicions of SARS and spark widespread panic. The world will
therefore anxiously watch if new outbreaks occur.